Field of the Invention
The present invention relates to a new category of analog multiplexers which are intended more particularly but not limitatively to be used for data acquisition during scientific observations made from space vessels.
More precisely, it deals with multiplexers which have been developed for a space program of the EUROPEAN SPACE AGENCY using infrared space observatories, called ISO program.
These observatories are cryogenic space vessels, resulting from space technology, which are equipped with image sensors formed by infrared (IR) sensitive cameras cooperating with an infrared telescope so as to make possible the scientific observation of the space environment. The use of ISO cryogenic vessels for this type of space observation is essential so as to reduce to a minimum the thermal noise of the sensors.
To achieve this, a reservoir filled with a coolant such as liquid helium (LHE) or liquid nitrogen (LN.sub.2) is installed in an ISO vessel, which allows the desired low temperature level to be obtained inside the vessel.
It is obvious that the lifetime of the coolant (and so the duration of the space mission) is inversely proportional to the thermal losses in the vessel, the major part of which is due to the thermal conductivity of the electric wires used for the connection of the sensors and actuators, disposed inside the cryogenic vessel, to the electronic monitoring devices placed outside the vessel.
It is known that the use of an analog multiplexer inside a cryogenic vessel greatly reduces the number of electric wires which pass through the interface separating the warm environment, existing outside the vessel, from the cold environment existing thereinside, (hereafter mention will be made more simply of warm/cold or cold/warm interface).
It has been calculated that the reduction of the number of wires thus obtained results in a reduction of the thermal losses such that the duration of an ISO program is increased by more than two months.
However, even a multiplexer dissipates electric power and generates heat, which affects the duration of a space mission so that, for the use of a multiplexer to form a real advantage, its power dissipation must be much lower than the reduction in thermal input due to the reduction of the number of wires providing the electric connections.
In order to justify the use of a multiplexer, it must have the following properties:
very low electric power consumption and more particularly less than 5 .mu.W/channel,
a very small number of control lines, and
the ability to operate in a temperature range between 4.2K (liquid helium temperature) and 77K (liquid nitrogen temperature).
Now, the constraints of the environment suggest using GaAs MESFETs (namely the technology using gallium arsenide, GaAs, as substrate instead of silicon in a junction FET variant called Schottky FET or MESFET).
In fact, on the one hand, CMOS cannot be used unless an improvement is made to the corresponding technology and, on the other hand, no bipolar technology seems to be able to withstand the stresses due to the low temperature involved in the use of liquid helium.
Insofar as the known types of multiplexers are concerned, FIGS. 1 and 2 accompany the present description show two configurations which are currently used in most applications.
FIG. 1 shows a binary tree multiplexer with 11 inputs and one output.
Among the 11 inputs there are eight input parameters I.sub.1 to I.sub.8 and three variables (or control signals) A, B, C. The output is designated by the letter O.
The switches are controlled switches, for example FET switches (using field effect transistors).
Generally, the standard configurations include N variables (or control signals) and 2.sup.N parameters I.sub.1 to I.sub.2.sup.N. Since for each control signal A, B, C there also exists its complement (or its inverse or negation) A, B, and C respectively--which are generally obtained by means of N inverters i (or NOT operators, also called NOT gates)--we then generally have 2N control lines.
Under these conditions it is obvious that, if the warm/cold interface is formed by the barrier designated schematically by the dot dash line .alpha., all the N inverters are disposed inside the cryogenic vessel and N lines are routed through this interface, whereas, if on the contrary, the warm/cold interfaces is formed by the barrier designated schematically by the line .beta., all the N inverters are disposed outside the cryogenic vessel and the number of lines which pass through the interface is equal to 2N.
Now, having dismissed the bipolar and CMOS technologies, for the reasons mentioned above, in favor of GaAs MESFET technology, it is obvious that, the interface represented by the line .alpha. should not be adopted in this case, that is, that the N inverters must not be disposed inside the cryogenic vessel, because GaAs inverters require a lot of supply power, well above 1 mW/gate.
Furthermore, it should also be taken into account that electric power is also dissipated in the ON (load) resistance of the enabled channel: since each channel has N switches in series (and so three in the case shown in FIG. 1), the total load resistance in three times higher than the ON (load) resistance of a single switch.
FIG. 2, a single switch array, shows an arrangement of individual switches controlled by a decoder d. The number of control signals and thus of control lines is N for 2N input parameters.
Since the power dissipated by the decoder is also relatively high in this case, the configuration shown in FIG. 2 cannot be adopted either, unless a very low ON (load) resistance is required while driving relatively high current loads.